Magnetic-electric switch element

ABSTRACT

A magnetic electric switch element particularly appropriate as a component for projectile fuzes or similar uses. A switch element having a magnetic field has a contact bridge serving as a switching relay inserted therein which can be loaded through pressure or when under pressure it can be unloaded, made of a metal alloy with a Curie temperature which varies through pressure and when applying a certain pressure it lowers the Curie temperature from ferromagnetic into a paramagnetic condition and inversely when decreasing the pressure while increasing the Curie temperature it changes from a paramagnetic condition into ferromagnetic.

o Ullltfld States Patent 1 1 3,569,876

[72] Inventors Carl-Otto Leiber [56] References Cited Orplid SH. 11, 7 stuttgal't-s ollnen bel'y; UNITED STATES PATENTS az Fati 68 2,789,184 4/1957 Matthews 335/146 [21 1 App] No $3 2,835,811 5/1958 Bruyning 336/110 3, 39, 83 19 6 Y Filed p 7,1969 2 7 3/ 6 etter 336/20(UX) [45] Patented Mar. 9, 1971 Primary Examiner-Harold Broome [32] v Priority Apr. 6, 1968 Att0meyWenderoth, Lind and Ponack [33] Germany P 7 v [3n 1 63 1300 ABSTRACT: A magnetic electric switch element particularly appropriate as a component for projectile fuzes or similar WIT uses. A switch element having a magnetic field has a contact [54] ya g% 3 s CH ELEMENT bridge serving as a switching relay inserted therein which can rawmg be loaded through pressure or when under pressure it can be [52] U.S.Cl. 335/2, unloaded, made of a metal alloy with a Curie temperature 335/196 which varies through pressure and when applying a certain [51] Int. Cl. H0lb pressure it lowers the Curie temperature from ferromagnetic [50] Field of Search 335/2, 3, into a paramagnetic condition and inversely when decreasing 85,185,196, 244, 296; 338/1 14; 336/20, 179, 1l0;3l7/81, 79 (PE) the pressure while increasing the Curie temperature it changes from a paramagnetic condition into ferromagnetic.

Patented- March 9, 1971 3,569,876

3 Sheets-Sheet l Fly INVENTORS CA -OTTO L' ER GU ER HEI By WMAM$$M Attorneys Patented "March 9, 1971- 3,569,876

5 Sheets-Sheet 2 7 Q E @I 3 IQ /A/\/// m I 6 5 Fly. 2

CA OTTO IBER GU ER H KE y WKAZM ATTORNEYS INVENTORS CARL-OTTO LEIBER GUNTHER HEIMKE ATTORNEYS imi 5 Sheets-Sheet 3 wwiwiwm MAGNETTC-ELECTRIIC SWliTCI-ll ELEMENT The invention relates to a magnetic-electric switch element which serves to release through pressure the switching operation, comprising primarily a magnetic circuit with a switching relay for the opening and closing of the circuit and a coil whose windings are in the magnetic field and in case of a change of the magnetic flux, for instance at the time of the break down or the building up of the field due to the opening or closing of the magnetic circuit are cut by lines of force of the field.

Such switch elements are particularly indicated for magnetic-electric fuzes for projectiles or similar articles, because because as described below they do not need any movable parts.

Magnetic fuzes are already known as such. They are based on the principle of generating a change of the magnetic flux in a coil due to mechanical opening or closing of a magnetic circuit or because it causes a mechanical relative motion between a coil and a source of magnetic lines of force, for example that of a permanent magnet. These known fuzes, the same as the purely mechanical fuzes, are provided with movable parts causing often expensive and faulty construction.

It is an object of the present invention to create a magnetic electric switch element for any purpose, which nevertheless is particularly appropriate as a component for projectile fuzes or similar uses, furthermore it is extremely simple in its construction and is practically maintenance free. Starting with a switch element described above the problem is solved by inserting in the magnetic field a contact bridge serving as a switching relay which can be loaded through pressure or when under pressure it can be unloaded, made of a metal alloy with a Curie temperature which varies through pressure and when applying a certain pressure it lowers the Curie temperature from ferromagnetic into a paramagnetic condition and inversely when decreasing the pressure while increasing the Curie temperature it changes from a paramagnetic condition into ferromagnetic.

Raw materials having the aforementioned properties are already known, and have been used as compensation raw materials for the construction of magnetic circuits. They are metal alloys with for instance 30 percent by weight Ni and 70 percent by weight Fe, which exist in cubic face centered modifications. These alloys can be set for instance at a Curie temperature of approximately 155 C. and show a lowering of the Curie point when pressure increases by 5 to 60 C./Kilobar. Thus with a pressure of around 30 Kilobar the Curie-temperature can be brought down to 0C.

With such an alloy exposed to room temperature a pressure increase of the above mentioned amount causes a change from ferromagnetic to a paramagnetic condition. As the aforementioned lowering of the Curie temperature is inversely proportional to the increase of pressure it it therefore possible by decreasing the pressure in the above mentioned amount to change the paramagnetic condition of the alloy into a ferromagnetic condition.

The composition of the aforementioned alloy can vary substantially from the above-mentioned percentage. For instance an alloy of 36 percent by weight Ni and 64 percent by weight Fe has proven to be very useful. Besides the alloys mentioned here, there are others which have Curie-temperature dependent on pressure. Of course such alloys can also be used in accordance with this invention.

In a particularly efficient type of the proposed switch elemerit it is advantageous that the windings of the coil which are cut when changing the magnetic field, should surround at least a part of the length of the contact bridge. Here it is advantageous if the contact bridge has a circular cross section and is enclosed concentrically by the windings of the coil.

Of course the coil can be put on any other place of the magnetic circuit, for instance, within the range of a magnetic yoke which closes the magnetic circuit made for instance of soft iron. However in this case a delay is caused by transmission loss of the eddy current in the soft magnetic conducting pieces of the yoke, which is avoided when setting the coil concentric to the contact bridge.

Another object of the invention is to install coaxially to the contact bridge a pressure device in order to produce pressure upon the contact bridge.

When the above described switching element is used as a fuze for projectiles, there are immediately available at the time of the impact of such projectiles upon hard objects pressures in the size mentioned. According to this invention in the case of such fuzes the impact pressure is used for opening the magnetic field, because due to the effect of the pressure the Curie temperature of the contact bridge is lowered and the raw material of the contact bridge changes from the ferromagnetic to a paramagnetic condition.

According to another object of the invention a permanent magnet is installed in the magnetic circuit to energize the circuit, whereby one of the poles is connected with one end of the contact bridge and the other pole with the other end of the contact bridge, then the magnetic energy stoppage of the magnetic circuitbefore the impact of the projectile is zero, then the magnetic circuit is closed and it finds itself in a condition of residual magnetism. When the contact bridge contained in the magnetic field made of an iron-nickel alloy passes from a ferro-to a paramagnetic condition at the time of the impact, the magnetic field opens, whereby an interruption of the magnetic flux lines occurs, so that a demagnetizing field appears,

This cuts the field of the coil and a voltage is induced in the coil which is available as fuze voltage and in a given case may be adequately increased.

Further objects of the invention include the features that the permanent magnet with one pole is located directly at one end of the contact bridge, while the other end of the contact bridge is connected with the other magnetic pole through a yoke preferably of soft iron. The permanent magnet and the contact bridge can be arranged coaxially with each other, or the permanent magnet can be composed of two plate magnets, which face each other, and are approximately at a right angle to one of the ends of the contact bridge, while the magnetic circuit has passed through two yoke parts preferably of soft iron which connect the other magnetic poles with the other end of the contact bridge.

A further object is a variation of the above mentioned description in that the permanent magnet can also be a ring circuit which encloses concentrically the contact bridge in a radial distance along its complete length whereby the poles are connected with the ends of the contact bridge through end plates which serve as guiding pieces made of soft iron.

With the above and other objects in view which will become apparent from the detailed description below, some preferred forms of the invention, particularly for use in projectile fuzes are shown in the drawings in which:

FIG. 1 is a schematic view of one form of the invention;

F KG. 2 is a modified form of the invention; and

FIG. 3 is a similar view of a third form of the invention.

FIG. I shows a magnetic circuit composed of a permanent magnet l, a soft magnetic yoke 2 and the contact bridge 3 which is sensitive to pressure and is composed of a Ni-Fe alloy. The permanent magnet is magnetized in the direction of the arrow. Coil 4 transforms the energy liberated at the moment of impact into electrical energy so that, if it is also intensified, it may be available as igniting or signal energy or for other purposes, for example the pressure wave P created at the time of the impact of a projectile is lead by the pressure device 5 onto the pressure sensitive contact bridge, which thereby changes from the ferromagnetic into the paramagnetic condition.

FIG. 2 shows another distribution of the elements of the magnetic CIRCUIT. The flux of the radially magnetized permanent magnet 6 is brought by a soft magnetic yoke 7 which simultaneously serves as fuze casing, from the contact bridge 8 made of a pressure sensitive Ni-Fe alloy, which thus closes the magnetic circuit. Coil 9 has the same purpose as coil 4 in FIG. 1. Also in this type the force of impact P is transmitted through pressure device it) onto the pressure sensitive contact bridge 3.

According to the distribution of FIG. 3 the flux of the axially magnetized permanent magnet ring ill is lead through the soft magnetic leading device 12 to the pressure sensitive contact bridge 13, thereby transferring the impact force P through the pressure device 143 at the time of impact. Coil 15 is again distributed around the pressure sensitive contact bridge. It has the same function as the coil in FIG. 1. When designing the circuit comprising permanent magnets, soft magnetic leading devices and a pressure sensitive contact bridge as to its magnetic qualities, the greatest possible changes of the magnetic energy due to the impact must be considered. The working point of the magnet has to approach at the time of the impact as much as possible the coercive field force, which means that it has to be cut quite strongly.

The most remarkable advantage of the fuze according to the invention is that it does not have any mechanically movable parts. This makes the fuze mechanically very stable. It makes it also very quick because no parts need to be accelerated for its striking.

If there existed the danger that the fuze could be heated above the Curie temperature of the Ni-Fe alloy, according to a further feature of the invention it would be impossible to obtain a break of the circuit by inserting a bimetal contact 16 (FIG. 3) into the electrical shunt conductance of the ignition coil. 1

The striking of the fuze in case of small impact forces P is obtainable through adequate pressure transmission of the pressure device, which transmits the impact wave onto the Ni- Fe alloy.

We claim:

1. An impact fuze for projectiles with a magnetic-electric switching member for releasing through pressure electric switching operations comprising a magnetic circuit having a magnetic field, a magnetic switch element located in said magnetic circuit for opening and closing said magnetic circuit, a permanent magnet arranged in said magnetic circuit for energization thereof, said permanent magnet comprising two plate magnets aligned to face each other approximately at a right angle to one end of said magnetic switch element and said magnetic circuit is closed through two yoke parts which connect the other magnetic poles with the other end of said magnetic switch element, a coil having windings in the area of said magnetic field which in a change of the magnetic flux are cut by the lines of force of said field, such as at the time of a breakdown or building up of said field, due to the opening or closing of said magnetic circuit, said magnetic switch element comprising a metal alloy with a Curie temperature variable with pressure, so that when a certain pressure increases and the Curie temperature decreases its condition changes from ferromagnetic to paramagnetic and inversely when pressure decreases and Curie temperature increases its condition changes from paramagnetic to ferromagnetic whereby such changes induce a voltage in the coil to serve as ignition voltage for the fuze.

2. An impact fuze for projectiles with a magnetic-electric switching member for releasing through pressure electric switching operations comprising a magnetic circuit having a magnetic field, a magnetic switch element located in said magnetic circuit for opening and closing said magnetic circuit, a permanent magnet arranged in said magnetic circuit for energization thereof, said permanent magnet comprising a ring magnet enclosing said magnetic switch element at a radial distance concentrically along its whole extension and whose poles are connected with the ends of said magnetic switch element through end plates serving as guiding devices, a coil having windings in the area of said magnetic field which in a change of the magnetic flux are cut by the lines of force of said field, such as at the time of a breakdown or building up of said field due to the opening or closing of said magnetic circuit, said magnetic switch eiement comprising a metal alloy with a Curie temperature variable with pressure, so that when a certain pressure increases and the Curie temperature decreases its condition changes from ferromagnetic to paramagnetic and inversely when pressure decreases and Curie temperature increases its condition changes from paramagnetic to ferromag netic whereby such changes induce a voltage in the coil to serve as ignition voltage for the fuze.

3. A pressure responsive magnetic-electric switching device comprising in combination a magnetic core element, an inductive electrical winding substantially surrounding said core element, said core element comprising a portion formed of a metal alloy with a Curie temperature variable with pressure, so that when a certain pressure increases and the Curie temperature decreases its condition changes from ferromagnetic to paramagnetic and inversely when the pressure decreases and the Curie temperature increases its condition changes from paramagnetic to ferromagnetic accompanying and related to a second order transition, whereby by an application of compressive force to said core material there occurs an alteration in the Curie temperature response, which is of a magnitude of about 5 to -6C./Kilobat and such changes induce a voltage in said winding when said device is subjected to a compression force.

4. A pressure responsive magnetic-electric switching device comprising in combination a magnetic core element, an inductive electrical winding substantially surrounding said core element, said core element comprising a portion formed of a metal alloy with a Curie temperature variable with pressure, so that when a certain pressure increases and therefore the Curie temperature decreases its condition changes from ferromagnetic to paramagnetic and inversely when the pressure decreases and therefore the Curie temperature increases its condition changes from paramagnetic to ferromagnetic accompanying and related to a second order transition, whereby by an application of compressive force to said core element there occurs an alteration in the Curie temperature response, and such changes induce a voltage in said winding, when said device is subjected to a compression force.

5. A magnetic-electric switching member for releasing through pressure electric switching operations comprising a magnetic circuit having a magnetic field, a magnetic switch element located in said magnetic circuit for opening and closing said magnetic circuit, a permanent magnet arranged in said magnetic circuit for energization thereof, said permanent magnet comprising two plate magnets aligned to face each other approximately at a right angle to one end of said magnetic switch element and said magnetic circuit is closed through two yoke parts which connect the other magnetic poles with the other end of said magnetic switch element, a coil having windings in the area of said magnetic field which in a change of the magnetic flux are cut by the lines of force of said field, such as at the time of a breakdown or building up of said field, due to the opening or closing of said magnetic circuit, said magnetic switch element comprising a metal alloy with a Curie temperature variable with pressure, so that when a certain pressure increases and the Curie temperature decreases its condition changes from ferromagnetic to paramagnetic and inversely when temperature decreases and Curie temperature increases its condition changes from paramagnetic to ferromagnetic.

6. A magnetic-electric switching member for releasing through pressure electric switching operations comprising a magnetic circuit having a magnetic field, a magnetic switch element located in said magnetic circuit for opening and closing said magnetic circuit, a permanent magnet arranged in said magnetic circuit for energization thereof, said permanent magnet comprising a ring magnet enclosing said magnetic switch element at a radial distance concentrically along its whole extension and whose poles are connected with the ends of said magnetic switch element through end plates serving as guiding devices, a coil having windings in the area of said magnetic field which in a change of the magnetic flux are cut by the lines of force of said field, such as at the time of a breakdown or building up of said field due to the opening or closing of said magnetic circuit, said magnetic switch element comprising a metal alloy with a Curie temperature variable with pressure, so that when a certain pressure increases and the Curie temperature decreases its condition changes from ferromagnetic to paramagnetic and inversely when pressure decreases and Curie temperature increases its condition changes from paramagnetic to ferromagnetic.

7. An impact fuze for projectiles'with a magnetic-electric switching member for releasing through pressure electric switching operations comprising a magnetic circuit having a magnetic field, a magnetic switch element located in said magnetic circuit for opening and closing said magnetic circuit, a coil having windings in the area of said magnetic field which in a charge of the magnetic flux are cut by the lines of force of said field, such as at the time of a breakdown or building up of said field due to the opening or closing'of said magnetic circuit, said magnetic switch element comprising a metal alloy with a Curie temperature variable with pressure, so that when a certain pressure increases and the Curie temperature decreases its condition changes from ferromagnetic to paramagnetic and inversely when pressure decreases and Curie temperature increases its condition changes from paramagnetic to ferromagnetic whereby such changes induce a voltage in the coil to serve as ignition voltage for the fuze, a permanent magnet set in said magnetic circuit for energization thereof, one of the poles of said magnet being connected with one end of said magnetic switch element and the other pole with the other end of said magnetic switch element, said permanent magnet comprising two plate magnets aligned to face each other approximately at a right angle to one end of said magnetic switch element and said magnetic circuit is closed through two yoke parts which connect the other magnetic poles with the other end of said magnetic switch element.

8. An impact fuze for projectiles according to claim 7 wherein said two yoke parts are soft iron.

9. An impact fuze for projectiles with a magnetic-electric switching member for releasing through pressure electric switching operations comprising a'magnetic circuit having a magnetic field, a magnetic switch element located in said magnetic circuit for opening and closing said magnetic circuit, a coil having windings in the area of said magnetic field which in a charge of the magnetic flux are cut by the lines of force of said field, such as at the time of a breakdown or building up of said field due to the opening or closing of said magnetic cir cuit, said magnetic switch element comprising a metal alloy with a Curie temperature variable with pressure, so that when a certain pressure increases and the Curie temperature decreases its condition changes from ferromagnetic to paramagnetic and inversely when pressure decreases and Curie temperature increases its condition changes from paramagnetic to ferromagnetic whereby such changes induce a voltage in the coil to serve as ignition voltage for the fuze, a permanent magnet set in said magnetic circuit for energization thereof, one of the poles of said magnet being connected with one end of said magnetic switch element and the other pole with the other end of said magnetic switch element, said permanent magnet being a ring magnet enclosing said magnetic switch element at a radial distance concentrically along its whole extension and whosepoles are connected with the ends of said magnetic switch element through end plates serving as guiding devices. 

1. An impact fuze for projectiles with a magnetic-electric switching member for releasing through pressure electric switching operations comprising a magnetic circuit having a magnetic field, a magnetic switch element located in said magnetic circuit for opening and closing said magnetic circuit, a permanent magnet arranged in said magnetic circuit for energization thereof, said permanent magnet comprising two plate magnets aligned to face each other approximately at a right angle to one end of said magnetic switch element and said magnetic circuit is closed through two yoke parts which connect the other magnetic poleS with the other end of said magnetic switch element, a coil having windings in the area of said magnetic field which in a change of the magnetic flux are cut by the lines of force of said field, such as at the time of a breakdown or building up of said field, due to the opening or closing of said magnetic circuit, said magnetic switch element comprising a metal alloy with a Curie temperature variable with pressure, so that when a certain pressure increases and the Curie temperature decreases its condition changes from ferromagnetic to paramagnetic and inversely when pressure decreases and Curie temperature increases its condition changes from paramagnetic to ferromagnetic whereby such changes induce a voltage in the coil to serve as ignition voltage for the fuze.
 2. An impact fuze for projectiles with a magnetic-electric switching member for releasing through pressure electric switching operations comprising a magnetic circuit having a magnetic field, a magnetic switch element located in said magnetic circuit for opening and closing said magnetic circuit, a permanent magnet arranged in said magnetic circuit for energization thereof, said permanent magnet comprising a ring magnet enclosing said magnetic switch element at a radial distance concentrically along its whole extension and whose poles are connected with the ends of said magnetic switch element through end plates serving as guiding devices, a coil having windings in the area of said magnetic field which in a change of the magnetic flux are cut by the lines of force of said field, such as at the time of a breakdown or building up of said field due to the opening or closing of said magnetic circuit, said magnetic switch element comprising a metal alloy with a Curie temperature variable with pressure, so that when a certain pressure increases and the Curie temperature decreases its condition changes from ferromagnetic to paramagnetic and inversely when pressure decreases and Curie temperature increases its condition changes from paramagnetic to ferromagnetic whereby such changes induce a voltage in the coil to serve as ignition voltage for the fuze.
 3. A pressure responsive magnetic-electric switching device comprising in combination a magnetic core element, an inductive electrical winding substantially surrounding said core element, said core element comprising a portion formed of a metal alloy with a Curie temperature variable with pressure, so that when a certain pressure increases and the Curie temperature decreases its condition changes from ferromagnetic to paramagnetic and inversely when the pressure decreases and the Curie temperature increases its condition changes from paramagnetic to ferromagnetic accompanying and related to a second order transition, whereby by an application of compressive force to said core material there occurs an alteration in the Curie temperature response, which is of a magnitude of about -5 to -6C./Kilobat and such changes induce a voltage in said winding when said device is subjected to a compression force.
 4. A pressure responsive magnetic-electric switching device comprising in combination a magnetic core element, an inductive electrical winding substantially surrounding said core element, said core element comprising a portion formed of a metal alloy with a Curie temperature variable with pressure, so that when a certain pressure increases and therefore the Curie temperature decreases its condition changes from ferromagnetic to paramagnetic and inversely when the pressure decreases and therefore the Curie temperature increases its condition changes from paramagnetic to ferromagnetic accompanying and related to a second order transition, whereby by an application of compressive force to said core element there occurs an alteration in the Curie temperature response, and such changes induce a voltage in said winding, when said device is subjected to a compression force.
 5. A magnetic-electric switching member for releasing throuGh pressure electric switching operations comprising a magnetic circuit having a magnetic field, a magnetic switch element located in said magnetic circuit for opening and closing said magnetic circuit, a permanent magnet arranged in said magnetic circuit for energization thereof, said permanent magnet comprising two plate magnets aligned to face each other approximately at a right angle to one end of said magnetic switch element and said magnetic circuit is closed through two yoke parts which connect the other magnetic poles with the other end of said magnetic switch element, a coil having windings in the area of said magnetic field which in a change of the magnetic flux are cut by the lines of force of said field, such as at the time of a breakdown or building up of said field, due to the opening or closing of said magnetic circuit, said magnetic switch element comprising a metal alloy with a Curie temperature variable with pressure, so that when a certain pressure increases and the Curie temperature decreases its condition changes from ferromagnetic to paramagnetic and inversely when temperature decreases and Curie temperature increases its condition changes from paramagnetic to ferromagnetic.
 6. A magnetic-electric switching member for releasing through pressure electric switching operations comprising a magnetic circuit having a magnetic field, a magnetic switch element located in said magnetic circuit for opening and closing said magnetic circuit, a permanent magnet arranged in said magnetic circuit for energization thereof, said permanent magnet comprising a ring magnet enclosing said magnetic switch element at a radial distance concentrically along its whole extension and whose poles are connected with the ends of said magnetic switch element through end plates serving as guiding devices, a coil having windings in the area of said magnetic field which in a change of the magnetic flux are cut by the lines of force of said field, such as at the time of a breakdown or building up of said field due to the opening or closing of said magnetic circuit, said magnetic switch element comprising a metal alloy with a Curie temperature variable with pressure, so that when a certain pressure increases and the Curie temperature decreases its condition changes from ferromagnetic to paramagnetic and inversely when pressure decreases and Curie temperature increases its condition changes from paramagnetic to ferromagnetic.
 7. An impact fuze for projectiles with a magnetic-electric switching member for releasing through pressure electric switching operations comprising a magnetic circuit having a magnetic field, a magnetic switch element located in said magnetic circuit for opening and closing said magnetic circuit, a coil having windings in the area of said magnetic field which in a charge of the magnetic flux are cut by the lines of force of said field, such as at the time of a breakdown or building up of said field due to the opening or closing of said magnetic circuit, said magnetic switch element comprising a metal alloy with a Curie temperature variable with pressure, so that when a certain pressure increases and the Curie temperature decreases its condition changes from ferromagnetic to paramagnetic and inversely when pressure decreases and Curie temperature increases its condition changes from paramagnetic to ferromagnetic whereby such changes induce a voltage in the coil to serve as ignition voltage for the fuze, a permanent magnet set in said magnetic circuit for energization thereof, one of the poles of said magnet being connected with one end of said magnetic switch element and the other pole with the other end of said magnetic switch element, said permanent magnet comprising two plate magnets aligned to face each other approximately at a right angle to one end of said magnetic switch element and said magnetic circuit is closed through two yoke parts which connect the other magnetic poles with the other end of said magnetic switch element.
 8. An imPact fuze for projectiles according to claim 7 wherein said two yoke parts are soft iron.
 9. An impact fuze for projectiles with a magnetic-electric switching member for releasing through pressure electric switching operations comprising a magnetic circuit having a magnetic field, a magnetic switch element located in said magnetic circuit for opening and closing said magnetic circuit, a coil having windings in the area of said magnetic field which in a charge of the magnetic flux are cut by the lines of force of said field, such as at the time of a breakdown or building up of said field due to the opening or closing of said magnetic circuit, said magnetic switch element comprising a metal alloy with a Curie temperature variable with pressure, so that when a certain pressure increases and the Curie temperature decreases its condition changes from ferromagnetic to paramagnetic and inversely when pressure decreases and Curie temperature increases its condition changes from paramagnetic to ferromagnetic whereby such changes induce a voltage in the coil to serve as ignition voltage for the fuze, a permanent magnet set in said magnetic circuit for energization thereof, one of the poles of said magnet being connected with one end of said magnetic switch element and the other pole with the other end of said magnetic switch element, said permanent magnet being a ring magnet enclosing said magnetic switch element at a radial distance concentrically along its whole extension and whose poles are connected with the ends of said magnetic switch element through end plates serving as guiding devices. 